MONGOID-2951 Mark attributes changed when accessed via #read_attribute / subscript operator (#5284)

* MONGOID-4726 proof of concept for indication of change using []

* MONGOID-4726 add test

* reformat

Author: neilshweky

lib/mongoid/attributes.rb

@@ -86,7 +86,9 @@ def has_attribute_before_type_cast?(name)
     def read_attribute(name)
       field = fields[name.to_s]
       raw = read_raw_attribute(name)
-      field ? field.demongoize(raw) : raw
+      value = field ? field.demongoize(raw) : raw
+      attribute_will_change!(name.to_s) if value.resizable?
+      value
     end
     alias :[] :read_attribute
 

spec/mongoid/attributes_spec.rb

@@ -2684,4 +2684,18 @@
       end
     end
   end
+
+  context "when modifiying a hash referenced with the [] notation" do
+    let(:church) { Church.create!(location: { x: 1 }) }
+
+    before do
+      church[:location].merge!(y: 2)
+      church.save!
+      church.reload
+    end
+
+    it "persists the updated hash" do
+      church.location.should == { "x" => 1, "y" => 2 }
+    end
+  end
 end